(Acoustic Sensor of Patent Document 1)
FIG. 1 is a plan view illustrating a structure of an acoustic sensor described in Patent Document 1. FIG. 2 is a sectional view taken on a line X-X in FIG. 1. In acoustic sensor 11, conductive diaphragm 14 (movable electrode film) is provided above a top surface of silicon substrate 13 that back chamber 12 vertically pierces, base unit 15 made of SiO2 is formed on the top surface of silicon substrate 13 so as to surround diaphragm 14, and contact layer 16 thinner than base unit 15 is formed in an area outside base unit 15.
Protective film 17 made of an insulating material (SiN) is formed on the whole top surface of silicon substrate 13. Protective film 17 includes dome 18 that is provided above diaphragm 14 so as to cover diaphragm 14, base coating unit 19 that is provided outside dome 18 while having an inverted V-shape in section, and flat unit 20 that is provided outside base coating unit 19. Fixed electrode film 21 is provided on a bottom surface of dome 18 in an area opposed to diaphragm 14, and a capacitor is constructed with diaphragm 14 and fixed electrode film 21 in order to convert an acoustic vibration into an electric signal. Base coating unit 19 covers base unit 15, and flat unit 20 covers the top surface of contact layer 16. Flat unit 20 covers silicon substrate 13 up to an edge of the top surface of silicon substrate 13.
Electrode pads 22 and 23 are provided on the top surface of flat unit 20, electrode pad 22 is electrically connected to diaphragm 14 through flat unit 20, and electrode pad 23 is electrically connected to fixed electrode film 21. Acoustic hole 24 is made in dome 18 and fixed electrode film 21 in order that the acoustic vibration passes through acoustic hole 24.
(Problem of Laser Dicing Property)
In the case that the acoustic sensor is prepared by a MEMS (Micro Electro Mechanical Systems) technology, the plural acoustic sensors are prepared on one wafer at one time, and the acoustic sensors on the wafer are divided into chips by dicing. At this point, when the dicing is performed by a method of dividing the wafer into the chips with a dicing blade, cooling pure water invades into the acoustic sensor, and troubles such as sticking of the diaphragm are possibly generated. Therefore, laser dicing is used to divide the acoustic sensors into the chips. In the laser dicing, the wafer is scanned with a laser beam along a dicing street (cutting band), and a silicon substrate is modified by the laser beam to form amorphous silicon, thereby dividing the wafer along the dicing street.
However, in the acoustic sensor described in Patent Document 1, because the whole top surface of the silicon substrate is covered with the protective film, the whole chip forming area of the wafer is covered with the protective film when the plural acoustic sensors are prepared on the wafer. For this reason, as illustrated in FIG. 3, in the case that wafer 25 is irradiated with laser beam 26 along the dicing street, wafer 25 is irradiated with laser beam 26 through protective film 17 made of SiN. As a result, a shift of a focal point of laser beam 26 or attenuation of laser beam intensity is generated, and possibly a problem is generated in the laser dicing. It is necessary to slow down a scan speed of laser beam 26 in order to prevent a dicing failure, which degrades throughput during production of the acoustic sensor.
(Problem of Suction Property with Suction Collet)
In the case that each acoustic sensor divided into the chip is mounted on a circuit substrate or a casing, the acoustic sensor is sucked and conveyed with a suction collet (pickup tool). FIG. 5 is a fragmentary plan view illustrating a state in which acoustic sensor 11 is sucked with suction collet 27, and FIG. 4 is a sectional view taken on a line Y-Y in FIG. 5. Vacuum suction hole 28 is made at a leading end of suction collet 27. In the case that acoustic sensor 11 is sucked, the leading end of suction collet 27 abuts on the top surface of acoustic sensor 11, and vacuum suction hole 28 is evacuated or brought into a negative pressure to suck acoustic sensor 11.
However, for acoustic sensor 11 in FIGS. 1 and 2, in the case that the leading end of suction collet 27 abuts on the top surface of acoustic sensor 11, the leading end of suction collet 27 abuts on the top surface of base coating unit 19 to generate a gap between vacuum suction hole 28 and flat unit 20 as illustrated in FIGS. 4 and 5. Therefore, air flows into suction collet 27 from vacuum suction hole 28, and acoustic sensor 11 cannot successfully be sucked with suction collet 27. As a result, sometimes acoustic sensor 11 cannot be sucked and lifted with suction collet 27, or sometimes acoustic sensor 11 is dropped during the conveyance.
(Acoustic Sensor of Patent Document 2)
In an acoustic sensor disclosed in Patent Document 2, although the base unit and base coating unit do not exist, the flat unit located outside the dome does not have an area wide enough to be sucked with the suction collet. In the acoustic sensor of Patent Document 2, the whole top surface of the silicon substrate is covered with the protective film. Therefore, even if the flat unit of the protective film is widened such that the flat unit can be sucked with the suction collet, the surface of the protective film is coarser than the top surface of the silicon substrate, and a suction force is insufficiently obtained.
Additionally, in the acoustic sensor of Patent Document 2, because the whole top surface of the silicon substrate is covered with the protective film, the whole chip forming area of the wafer is covered with the protective film in the case that the plural acoustic sensors are prepared on the wafer at one time. Therefore, in the case that the wafer is scanned with the laser beam along the dicing street by the laser dicing, similarly to the acoustic sensor of Patent Document 1, the shift of the focal point of the laser beam or the attenuation of the laser beam intensity is generated, and the problem is generated in the laser dicing.